Analyzing samples representative of downhole fluids is an important aspect of determining the quality and economic value of a hydrocarbon formation.
Present day operations obtain an analysis of downhole fluids usually through wireline logging using a formation tester such as the MDT™ tool of Schlumberger Oilfield Services. However, more recently, it was suggested to analyze downhole fluids either through sensors permanently or quasi-permanently installed in a wellbore or through one or more sensors mounted on the drillstring. The latter method, where successfully implemented, has the advantage of obtaining data while drilling, whereas the former installation could provide additional value as part of a control system for wellbores and hydrocarbon production therefrom.
To obtain an estimate of the composition of downhole fluids, the MDT tools uses an optical probe to estimate the amount of hydrocarbons in the samples collected from the formation. Other sensors use resistivity measurements to discern various components of the formations fluids.
General downhole measurement tools for oilfield applications are known as such. Examples of such tools are found in the U.S. Pat. Nos. 6,023,340; 5,517,024; and 5,351,532 or in the International Patent Application WO 99/00575. An example of a probe for potentiometric measurements of ground water reservoirs is further published as: Solodov, I. N., Velichkin, V. I., Zotov, A. V. et al. “Distribution and Geochemistry of Contaminated Subsurface Waters in Fissured Volcanogenic Bed Rocks of the Lake Karachai Area, Chelyabinsk, Southern Urals” in: Lawrence Berkeley Laboratory Report 36780/UC-603 (1994b), RAC-6, Ca, USA.
If such devices were enabled to determine downhole trace amounts of light hydrocarbon molecules such as methane, they could offer an advance warning system for gas kicks, which is a major safety concern for drilling process. They could also provide valuable information regarding the location, distribution and composition of hydrocarbon reservoirs during logging operations.
The simple structure of methane and other gaseous, aliphatic hydrocarbons (≦C5H12) means that only very limited potential reactions are available for these molecules. A particularly important reaction is their oxidative conversion into the corresponding alcohols. It is known that microbes existing in sub-surface reservoirs perform such conversion, in situ, via highly specific catalytic interactions involving embedded enzymes. A summary describing these microbes is found for example in: M. T. Madigan and B. L. Marrs, “Extremophiles”, Sci. Am., 82-87 (1997).
The oxidative conversion chemistry of methane usually takes three major routes, two of which end up, ultimately, as CO2 and H2O via one of the following sequences:CH4→CH3OH→CH2HO→CHOOH→CO2+H2O, or  (1)CH4→C2H6→C2H4, or  (2)CH4→CO2+H2O  (3)
The most relevant and best understood reaction of methane so far is its partial oxidative conversion into methanol (reaction (1)), which is widely regarded as one of nature's greatest challenges to mankind, mainly due to the economic significance of the reaction product. Though thermodynamically feasible (ΔG0=−111.2 kJ mol−1), the reaction does not happen spontaneously to any observable extent under ambient conditions. Theoretical calculations show that neither elevated temperatures nor pressures result in substantial change in the free energy of the reaction.
At issue is the activation of the C—H bond, which is stronger in these gaseous hydrocarbons than in any other organic molecules. In nature, the activation process is enabled by the catalytic centre of the enzyme methane monooxygenase (MMO), in the presence of dioxygen, which, in turn, is activated by nicotinamide adenine dinucleotide hydride ion (NADH).
The catalytic centers in MMO that are responsible for the process of activating the C—H bond are, for soluble MMO, a diiron centre as described for example by L. Shu, J. C. Nesheim, K. Kauffmann, E. Munck, J. D. Lipscomb, L. Que, Jr., “An Fe2IVO2 diamond core structure for the key intermediate Q of methane monooxygenase” Science, 275, 515-518 (1997) and for membrane-bound MMO, a tricopper cluster as described in H-H. T. Nguyen, A. K. Shiemke, S. J. Jacobs, B. J. Hales, M. E. Lidstrom and S. I. Chan, “The nature of the copper ions in the membranes containing the particulate methane monooxygenase from methylococcus capsulatus (Bath)”, Biol. Chem., 269, 14995-15005 (1994).
Another known approach to methane activation is through an electro-chemical system which enables dioxygen to diffuse through a 130 μm thick silver membrane, which is controlled at a sufficiently negative potential to reduce the former into atomic oxygen, and react with methane on the other side. Sufficient dioxygen will then react with CH3. radicals to form, via a complex chain reactions, methanol as against possible coupling dimer products. Details of this method are described by A. G. Anshits, A. N. Shigapov, S. N. Vereshchagin and V. N. Shevin, “C2 hydrocarbon formation from methane on silver membrane”, Catal. Today, 6, 593-600 (1990)
An electro-chemical cell containing an iron-porphyrin deposited graphite cathode is known to convert light hydrocarbons into corresponding alcohols with considerable efficiency and described in: A. M. Khenin and A. E. Shilov, “Biomimetic alkane oxidation in the presence of iron complexes”, New J. Chem., 13, 659-667 (1989).
Applications of a number of transition metal compounds as catalysts for the activation of methane partial oxidation are summarized in: A. D. Ryabov, “Mechanism of intermolecular activation of C—H bonds in transition metal complexes”, Chem. Rev., 90, 403-424 (1990).
Periana et al. (J. H. Dygos, R. A. Periara, D. J. Taube, E. R. Evitt, D. G. Loffler, P. R. Wentrcek, Voss and T. Masuda, “A mercury-catalyzed, high-yield system for the oxidation of methane to methanol”, Science, 259, 340-343 (1993)) reported a homogeneous catalytic system which led to a high yield of methanol from methane partial oxidation via methyl disulfate. The net reaction catalyzed by either mercury, thallium, palladium, platinum or gold ions is the oxidation of methane, via an electrophilic displacement mechanism, involving concentrated sulfuric acid to produce ˜43% methyl disulfate. The subsequent hydrolysis resulted in methanol and simultaneous re-generation of the active form of the catalyst. The same group also most recently reported a one-step conversion of methane to acetic acid catalyzed by Pd in an acidic medium in: R. A. Periana, O. Mironov, D. Taube, G. Bhalla and C. J. Jones, “Catalytic, oxidative condensation of CH4 to CH3COOH in one step via CH activation”, Science, 301, 814-818 (2003).
It is also known that the C—H bond can also be activated by photolysis.
Various methane detection devices exist. In U.S. Pat. No. 4,282,487, a hydrocarbon detection system is described for the application of subsea oil and gas production. The system is based on a pair of inductive elements that are electrically coupled to the surrounding seawater. Displacement of conductive seawater by escaping hydrocarbons affects the interactions between the inductive elements, leading to a hydrocarbon-responsive output signal.
A wellsite alarm system designed to detect a sudden influx of hydrocarbon gases (“kicks”) while drilling oil wells is described in U.S. Pat. No. 4,802,143. The system is based on a thermal conductivity sensor which responds to an abnormal amount of gas, presumably light hydrocarbons, in the mud, oil and gas mixture passing the sensor interface. Mounted with an acoustic impulse generator, this sensor operates at a predetermined threshold of gas concentration.
In the U.S. Pat. No. 5,351,532 there is described an in-hole probe to measure hydrocarbon concentrations in drilling fluids around the drill string. Ultra-violet irradiation is directed into a detection chamber, where the sensor apparatus determines the fluorescent energy radiating from ethanol-soluble, aromatic hydrocarbons. A mechanism is introduced to distinguish between the fluorescent signals originating from sub-surface fluids and those caused by the diesel oil in drilling mud.
Whilst there are numerous examples of catalytic oxidation of methane, and a number of methods for detecting methane, it is an object of the present invention to provide a sensor for aliphatic hydrocarbons of low molecular weight. It is an object to make use of reaction processes for the purpose of monitoring hydrocarbon concentration, particularly for the purpose of determining methane concentration at subterranean locations. It is a further object of the present invention to provide downhole sensors and sensing methods for methane.